Question

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: The amino acid sequences of Histones H3 and H4 are not highly conserved.
Reason R: Nearly all of the amino acids in a histone molecule are engaged in an interaction with either D or another histone.

• A. Both A and R are correct and R is the correct explation of A
• B. Both A and R are correct but R is NOT the correct explation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

Histones H3 and H4 are among the most evolutiory conserved proteins in all eukaryotic organisms!
Reason: Almost every amino acid residue makes crucial contacts with D or other histones within the nucleosome octamer.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question examines the evolutiory conservation of histone proteins (H3 and H4) and the structural basis of their molecular interactions within the nucleosome core.

Step 2: Key Formula or Approach:
Histones are basic, positively charged proteins (rich in Lysine and Arginine) that package eukaryotic D into nucleosomes. Core histones H3 and H4 undergo extreme selective constraint during evolution due to tight structural interactions.

Step 3: Detailed Explation:

• Alysis of Assertion A: Histone proteins H3 and H4 are among the most extraordirily conserved proteins known across all eukaryotic evolution. For instance, histone H4 from pea seedlings differs from bovine histone H4 by only 2 amino acid residues out of 102 positions across over a billion years of evolutiory divergence. Thus, stating that H3 and H4 are *not* highly conserved is completely false. Therefore, Assertion A is incorrect.

• Alysis of Reason R: Virtually every amino acid side chain in core histones is involved in essential contacts: binding negatively charged D phosphodiester backbones or participating in protein-protein interactions within the H3-H4 tetramer and H2A-H2B dimers. Any mutation in these residues typically disrupts nucleosome assembly and is lethal, explaining their high conservation. Thus, Reason R is correct.

Step 4: Fil Answer:
Assertion A is incorrect because Histones H3 and H4 are extremely conserved, but Reason R is correct.